Gundogan hopes the weekend defeat by Leeds – when manager Pep Guardiola made seven changes – could help City.
"Maybe it was a warning or a signal for us," he said, with City going to Dortmund defending a one goal lead. "If it turns out to be like that. We will probably know after the game tomorrow. I'll take that defeat then. But of course we went into that game also to win it.
"Now we face a different competition, a different team and we are totally focused on that and then we will focus on the other competitions.
"Going to Dortmund, facing Borussia Dortmund knowing that there is no crowd feels like it's a bit of a disadvantage for Borussia Dortmund because I played there five years and I know how incredible the crowd is and the support is and how loud it can be also in that stadium.
"For them, knowing there is no crowd to support them - and the same for us in our stadium - there is something missing in football at the moment and we can't wait to have crowds back in stadiums."